The present invention relates to a process for preparing ammonium paratungstate tetrahydrate and also ammonium paratungstate tetrahydrate having a very high purity.
Ammonium paratungstate hydrates (hereinafter referred to as APTs) are known intermediates for producing tungsten metal, tungsten-containing catalysts or hard materials based on tungsten, for example tungsten carbides, or sputtering targets. The latter application requires a particularly high purity of the APT.
The preparation of highly pure APT is carried out essentially via acid or alkali digestion or fusion of tungsten-containing concentrates or tungsten scrap with subsequent purification stages involving precipitation processes and liquid-liquid extraction. The purified solution is generally concentrated by evaporation, resulting in APT finally crystallizing out.
According to the prior art, tungsten ore concentrates (e.g. scheelite, tungstite) and W-containing scrap are digested with sodium carbonate and/or sodium hydroxide. The Na tungstate solution formed is freed of Si, P, F, Mo, As, Sb, Bi, Co etc. in a preliminary purification in a number of steps by means of precipitation or ion-exchange processes. The sodium tungstate solution is subsequently converted into ammonium tungstate solution by liquid-liquid extraction using amine-containing organic phases. APT is prepared from this solution by evaporative crystallization.
As an alternative, the purification and conversion process of the digestion solutions is, for example, carried out industrially by means of ion-exchange resins in China or Uzbekistan. Here, tungsten and molybdenum are taken up by an exchanger while impurities such as Si, P and As remain in the raffinate. Tungsten is subsequently separated from molybdenum by selective elution. APT is likewise produced from the tungsten-containing eluate by evaporative crystallization. Typical impurity contents of the APT products prepared in this way are shown in Table 2 in the examples.
Products prepared in this way typically have a purity of from 99.9 to 99.99% by weight (3-4N).
Detailed descriptions of previously known processes for preparing APT are described in DE-B-1,244,141, DE-A-196 28 816, DE-C-197 24 183 and U.S. Pat. No. 4,115,513 and also in the publications “Anion exchange method for preparation of ammonium paratungstate from soda solutions, Kholmogorov, Vaneeva, Yurkevich, Tsvetnye Metally (Moscow, Russian Federation) 7/1978, pp. 59-62”, “Use of extraction by amines during the preparation of ammonium paratungstate, Petrov, Maslenitskii, Davydova, Tsvetnye Metally (Moscow, Russian Federation) 3/1974, pp. 38-39”, “New process for production of high purity ammonium paratungstate from tungsten siimes with high content of impurities, Zhang, Gong, Huang, Huang, Zhongnan Kuangye Xueyuan Xuebao (1990), 21(4), 389-96”, “Preparation of Paratungstate from a Sodium Tungstate-Sodium Chloride Phase, Raddatz, Gomes, Carnahan, Rep. Invest. US, Bur. Mines (1988), RI 9165” and also in the book “TUNGSTEN Properties Chemistry, Technology of the Element, Alloys, and Chemical Compounds (ISBN 0-306-45053-4, 1999), Lassner, Schubert, pp. 184-212.
In typical processes of the prior art, W concentrate is digested with sodium hydroxide or sodium carbonate and the resulting solution is freed of impurities such as P, As, Si, V and Mo in a preliminary purification with addition of Mg salts, Al salts and sodium hydrogensulfide. The sodium tungstate solution is subsequently purified by means of liquid-liquid extraction using an organic phase (e.g. containing 7-10% by weight of diisotridecylamine, 10% by weight of isodecanol, balance: petroleum spirit) and converted into ammonium tungstate solution. This solution is fractionally evaporated and the APT formed is separated off, washed and dried. Typical impurity contents of an APT product prepared in this way are shown in Table 1 in the examples.
A great disadvantage of the previously known processes is that the purity of the products which can be prepared thereby is insufficient for specific applications, e.g. for sputtering targets. In addition, the APT manufacturing costs are very high because of the high energy consumption of the evaporative crystallization.